[FLINK-4265] [dataset api] Add a NoOpOperator

Adds a NoOpOperator which is unwound in OperatorTranslation.translate.
This will be first used by Gelly as a placeholder to support implicit
operator reuse.

This closes apache#2294

flink-java/src/main/java/org/apache/flink/api/java/operators/NoOpOperator.java

@@ -0,0 +1,51 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.flink.api.java.operators;
+
+import org.apache.flink.annotation.Internal;
+import org.apache.flink.api.common.typeinfo.TypeInformation;
+import org.apache.flink.api.java.DataSet;
+import org.apache.flink.util.Preconditions;
+
+/**
+ * This operator will be ignored during translation.
+ *
+ * @param <IN> The type of the data set passed through the operator.
+ */
+@Internal
+public class NoOpOperator<IN> extends DataSet<IN> {
+
+	private DataSet<IN> input;
+
+	public NoOpOperator(DataSet<IN> input, TypeInformation<IN> resultType) {
+		super(input.getExecutionEnvironment(), resultType);
+
+		this.input = input;
+	}
+
+	public DataSet<IN> getInput() {
+		return input;
+	}
+
+	public void setInput(DataSet<IN> input) {
+		Preconditions.checkNotNull(input);
+
+		this.input = input;
+	}
+}

flink-java/src/main/java/org/apache/flink/api/java/operators/OperatorTranslation.java

@@ -19,24 +19,23 @@
 package org.apache.flink.api.java.operators;
 
 import org.apache.flink.annotation.Internal;
-import org.apache.flink.api.common.Plan;
-import org.apache.flink.api.java.DataSet;
-
-import java.util.ArrayList;
-import java.util.HashMap;
-import java.util.List;
-import java.util.Map;
-
 import org.apache.flink.api.common.InvalidProgramException;
+import org.apache.flink.api.common.Plan;
 import org.apache.flink.api.common.operators.AbstractUdfOperator;
 import org.apache.flink.api.common.operators.BinaryOperatorInformation;
 import org.apache.flink.api.common.operators.GenericDataSinkBase;
 import org.apache.flink.api.common.operators.Operator;
 import org.apache.flink.api.common.operators.UnaryOperatorInformation;
 import org.apache.flink.api.common.operators.base.BulkIterationBase;
 import org.apache.flink.api.common.operators.base.DeltaIterationBase;
+import org.apache.flink.api.java.DataSet;
 import org.apache.flink.configuration.Configuration;
 
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
 @Internal
 public class OperatorTranslation {
 
@@ -70,7 +69,10 @@ private <T> GenericDataSinkBase<T> translate(DataSink<T> sink) {
 
 
 	private <T> Operator<T> translate(DataSet<T> dataSet) {
-
+		while (dataSet instanceof NoOpOperator) {
+			dataSet = ((NoOpOperator<T>) dataSet).getInput();
+		}
+
 		// check if we have already translated that data set (operation or source)
 		Operator<?> previous = (Operator<?>) this.translated.get(dataSet);
 		if (previous != null) {